Semiconductor device, method for manufacturing semiconductor device, inverter circuit, drive device, vehicle, and elevator

ABSTRACT

A semiconductor device according to an embodiment includes: a silicon carbide layer; a silicon oxide layer; and a region disposed between the silicon carbide layer and the silicon oxide layer and having a nitrogen concentration equal to or more than 1×10 21  cm −3 . Nitrogen concentration distribution in the silicon carbide layer, the silicon oxide layer, and the region have a peak in the region, a nitrogen concentration at a position 1 nm away from the peak to the side of the silicon oxide layer is equal to or less than 1×10 18  cm −3 , and a carbon concentration at the position is equal to or less than 1×10 18  cm −3 .

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2020-049316, filed on Mar. 19, 2020, theentire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a semiconductor device,a method for manufacturing a semiconductor device, an inverter circuit,a drive device, a vehicle, and an elevator.

BACKGROUND

Silicon carbide (SiC) is expected as a material for next-generationsemiconductor devices. As compared with silicon (Si), silicon carbidehas superior physical properties such as a threefold band gap,approximately tenfold breakdown field strength, and approximatelythreefold thermal conductivity. By using these characteristics, asemiconductor device in which low loss and high-temperature operationcan be implemented can be realized.

However, for example, when a metal oxide semiconductor field effecttransistor (MOSFET) is formed using silicon carbide, there is a problemthat carrier mobility decreases or a threshold voltage changes. One offactors causing the decrease in the carrier mobility or the change inthe threshold voltage is considered to be carbon defects existing in agate insulating layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a semiconductor deviceaccording to a first embodiment;

FIG. 2 is a diagram showing a crystal structure of a SiC semiconductor;

FIG. 3 is a diagram showing an element concentration distribution of thesemiconductor device according to the first embodiment;

FIG. 4 is a process flow diagram of a method for manufacturing thesemiconductor device according to the first embodiment;

FIG. 5 is a diagram illustrating carbon defects;

FIG. 6 is a process flow diagram of a method for manufacturing asemiconductor device according to a comparative example;

FIG. 7 is a diagram showing an element concentration distribution of thesemiconductor device according to the comparative example;

FIG. 8 is a process flow diagram of a method for manufacturing asemiconductor device according to a second embodiment;

FIG. 9 is a process flow diagram of a method for manufacturing asemiconductor device according to a third embodiment;

FIG. 10 is a process flow diagram of a method for manufacturing asemiconductor device according to a fourth embodiment;

FIG. 11 is a process flow diagram of a method for manufacturing asemiconductor device according to a fifth embodiment;

FIG. 12 is a process flow diagram of a method for manufacturing asemiconductor device according to a sixth embodiment;

FIG. 13 is a schematic cross-sectional view of a semiconductor deviceaccording to a seventh embodiment;

FIG. 14 is a schematic cross-sectional view of a semiconductor deviceaccording to an eighth embodiment;

FIG. 15 is a schematic diagram of a drive device according to a ninthembodiment;

FIG. 16 is a schematic diagram of a vehicle according to a tenthembodiment;

FIG. 17 is a schematic diagram of a vehicle according to an eleventhembodiment; and

FIG. 18 is a schematic diagram of an elevator according to a twelfthembodiment.

DETAILED DESCRIPTION

A semiconductor device according to an embodiment includes: a siliconcarbide layer; a silicon oxide layer; and a region disposed between thesilicon carbide layer and the silicon oxide layer and having a nitrogenconcentration equal to or more than 1×10²¹ cm⁻³. Nitrogen concentrationdistribution in the silicon carbide layer, the silicon oxide layer, andthe region have a peak in the region, a nitrogen concentration at aposition 1 nm away from the peak to the side of the silicon oxide layeris equal to or less than 1×10¹⁸ cm⁻³, and a carbon concentration at theposition is equal to or less than 1×10¹⁸ cm⁻³.

Hereinafter, embodiments of the present disclosure will be describedwith reference to the drawings. In the following description, the sameor similar members are denoted by the same reference numerals and thedescription of the members described once is appropriately omitted.

In addition, in the following description, notations n⁺, n, n⁻, p⁺, p,and p⁻ represent the relative magnitudes of impurity concentrations inrespective conductive types. That is, an n-type impurity concentrationof n⁺ is relatively higher than an n-type impurity concentration of nand an n-type impurity concentration of n⁻ is relatively lower than then-type impurity concentration of n. In addition, a p-type impurityconcentration of p⁺ is relatively higher than a p-type impurityconcentration of p and a p-type impurity concentration of p⁻ isrelatively lower than the p-type impurity concentration of p. The n⁺type and the n⁻ type may be simply described as the n types and the p⁺type and the p⁻ type may be simply described as the p types. An impurityconcentration of each region is represented by, for example, a value ofan impurity concentration of a center portion of each region, unlessotherwise specified.

The impurity concentration can be measured by secondary ion massspectrometry (SIMS), for example. In addition, the relative magnitude ofthe impurity concentration can be determined from the magnitude of acarrier concentration obtained by scanning capacitance microscopy (SCM),for example. In addition, a distance such as a width and a depth of animpurity region can be obtained by SIMS, for example. In addition, thedistance such as the width and the depth of the impurity region can beobtained from an SCM image, for example.

A depth of a trench, a thickness of an insulating layer, and the likecan be measured on an SIMS or transmission electron microscope (TEM)image, for example.

Bonding states of carbon atoms, nitrogen atoms, and oxygen atoms in asilicon carbide layer can be identified by using X-ray photoelectronspectroscopy (XPS method).

First Embodiment

A semiconductor device according to a first embodiment includes: asilicon carbide layer; a silicon oxide layer; and a region disposedbetween the silicon carbide layer and the silicon oxide layer and havinga nitrogen concentration equal to or more than 1×10²¹ cm⁻³. Nitrogenconcentration distribution in the silicon carbide layer, the siliconoxide layer, and the region have a peak in the region, a nitrogenconcentration at a position 1 nm away from the peak to the side of thesilicon oxide layer is equal to or less than 1×10¹⁸ cm⁻³, and a carbonconcentration at the position is equal to or less than 1×10¹⁸ cm⁻³.

FIG. 1 is a schematic cross-sectional view of the semiconductor deviceaccording to the first embodiment. The semiconductor device is a MOSFET100. The MOSFET 100 is a double implantation MOSFET (DIMOSFET) in whicha p-well and a source region are formed by ion implantation. Further,the MOSFET 100 is an n-channel MOSFET using electrons as carriers.

The MOSFET 100 includes a silicon carbide layer 10, a gate insulatinglayer 28 (silicon oxide layer), a gate electrode 30, an interlayerinsulating film 32, a source electrode 34, a drain electrode 36, and aninterface termination region 40 (region).

The silicon carbide layer 10 includes a drain region 12, a drift region14, a p-well region 16, a source region 18, and a p-well contact region20.

The silicon carbide layer 10 is, for example, single crystal of 4H—SiC.The silicon carbide layer 10 is disposed between the source electrode 34and the drain electrode 36.

FIG. 2 is a diagram showing a crystal structure of a SiC semiconductor.A typical crystal structure of the SiC semiconductor is a hexagonalcrystal system such as 4H—SiC. One of faces (top faces of a hexagonalcolumn) with a c-axis along an axial direction of the hexagonal columnas a normal is a (0001) face. A face equivalent to the (0001) face isreferred to as a silicon face (Si face) and expressed as a {0001} face.Silicon atoms (Si) are arranged on an outermost face of the siliconface.

The other of the faces (top faces of the hexagonal column) with thec-axis along the axial direction of the hexagonal column as the normalis a (000-1) face. A face equivalent to the (000-1) face is referred toas a carbon face (C face) and expressed as a {000-1} face. Carbon atoms(C) are arranged on an outermost face of the carbon face.

On the other hand, a side face (column face) of the hexagonal column isan m face to be a face equivalent to a (1-100) face, that is, a {1-100}face. Further, a face passing through a pair of ridge lines not adjacentto each other is an a face to be a face equivalent to a (11-20) face,that is, a {11-20} face. Both the silicon atoms (Si) and the carbonatoms (C) are arranged on outermost faces of the m face and the a face.

Hereinafter, a case where a face of the silicon carbide layer 10 is aface inclined by an angle equal to or more than 0 degrees and equal toor less than 8 degrees with respect to the silicon face and a back faceis a face inclined by an angle equal to or more than 0 degrees and equalto or less than 8 degrees with respect to the carbon face will bedescribed as an example. The face of the silicon carbide layer 10 has anoff angle equal to or more than 0 degrees and equal to or less than 8degrees with respect to the silicon face.

The drain region 12 is n⁺-type SiC. The drain region 12 includesnitrogen (N) as n-type impurities, for example. An n-type impurityconcentration of the drain region 12 is, for example, equal to or morethan 1×10¹⁸ cm⁻³ and equal to or less than 1×10²¹ cm⁻³.

The drift region 14 is provided on the drain region 12. The drift region14 is n⁻-type SiC. The drift region 14 includes nitrogen as n-typeimpurities, for example.

An n-type impurity concentration of the drift region 14 is lower thanthe n-type impurity concentration of the drain region 12. The n-typeimpurity concentration of the drift region 14 is, for example, equal toor more than 1×10¹⁵ cm⁻³ and equal to or less than 2×10¹⁶ cm⁻³.

The drift region 14 is, for example, an SiC epitaxial growth layerformed on the drain region 12 by epitaxial growth.

A thickness of the drift region 14 is, for example, equal to or morethan 5 μm and equal to or less than 100 μm.

The p-well region 16 is provided on a partial face of the drift region14. The p-well region 16 is p-type SiC. The p-well region 16 includesaluminum (Al) as p-type impurities, for example. A p-type impurityconcentration of the p-well region 16 is, for example, equal to or morethan 1×10¹⁶ cm⁻³ and equal to or less than 1×10²⁰ cm⁻³.

A depth of the p-well region 16 is, for example, equal to or more than0.4 μm and equal to or less than 0.8 μm. The p-well region 16 functionsas a channel region of the MOSFET 100.

The source region 18 is provided on a partial face of the p-well region16. The source region 18 is n⁺-type SiC. The source region 18 includesphosphorus (P) as n-type impurities, for example. An n-type impurityconcentration of the source region 18 is, for example, equal to or morethan 1×10¹⁸ cm⁻³ and equal to or less than 1×10²² cm⁻³.

A depth of the source region 18 is shallower than the depth of thep-well region 16. The depth of the source region 18 is, for example,equal to or more than 0.2 μm and equal to or less than 0.4 μm.

The p-well contact region 20 is provided on a partial face of the p-wellregion 16. The p-well contact region 20 is provided on the side of thesource region 18. The p-well contact region 20 is p⁺-type SiC.

The p-well contact region 20 includes aluminum as p-type impurities, forexample. A p-type impurity concentration of the p-well contact region 20is, for example, equal to or more than 1×10¹⁸ cm⁻³ and equal to or lessthan 1×10²² cm⁻³.

A depth of the p-well contact region 20 is shallower than the depth ofthe p-well region 16. The depth of the p-well contact region 20 is, forexample, equal to or more than 0.2 μm and equal to or less than 0.4 μm.

The gate insulating layer 28 is provided between the silicon carbidelayer 10 and the gate electrode 30. The gate insulating layer 28 isprovided between the drift region 14 and the p-well region 16 and thegate electrode 30. The gate insulating layer 28 is provided on the driftregion 14 and the p-well region 16. The gate insulating layer 28 iscontinuously formed on the faces of the drift region 14 and the p-wellregion 16.

The gate insulating layer 28 is, for example, silicon oxide. The gateinsulating layer 28 is an example of a silicon oxide layer.

A thickness of the gate insulating layer 28 is, for example, equal to ormore than 30 nm and equal to or less than 100 nm. The gate insulatinglayer 28 functions as a gate insulating layer of the MOSFET 100. Thethickness of the gate insulating layer 28 is, for example, equal to ormore than 40 nm and equal to or less than 50 nm.

The interface termination region 40 is disposed between the siliconcarbide layer 10 and the gate insulating layer 28. The interfacetermination region 40 is disposed between the drift region 14 and thegate insulating layer 28, and the p-well region 16 and the gateinsulating layer 28. The interface termination region 40 includesnitrogen (N) as a termination element terminating a dangling bond of thesilicon carbide layer 10. The interface termination region 40 is anexample of a region.

A nitrogen concentration of the interface termination region 40 is equalto or more than 1×10²¹ cm⁻³.

FIG. 3 is a diagram showing an element concentration distribution of thesemiconductor device according to the first embodiment. FIG. 3 is adiagram showing element concentration distributions in the gateinsulating layer 28, the interface termination region 40, and thesilicon carbide layer 10. FIG. 3 shows concentration distributions ofnitrogen and carbon.

The nitrogen concentration distribution has a peak in the interfacetermination region 40. A nitrogen concentration at the peak is, forexample, equal to or more than 1×10²² cm⁻³. A full width at half maximumwith respect to the peak of the nitrogen concentration distribution is,for example, equal to or less than 1 nm. Nitrogen is segregated at aninterface between the silicon carbide layer 10 and the gate insulatinglayer 28.

The nitrogen concentration at a position X to be 1 nm away from the peakof the nitrogen concentration distribution to the side of the gateinsulating layer 28 is equal to or less than 1×10¹⁸ cm⁻³. The nitrogenconcentration is preferably equal to or less than 1×10¹⁷ cm⁻³ and morepreferably equal to or less than 1×10¹⁶ cm⁻³.

Nitrogen in the interface termination region 40 substitutes outermostcarbon atoms of the silicon carbide layer 10. Nitrogen in the interfacetermination region 40 is tri-coordinated with the silicon carbide layer.In other words, nitrogen atoms are at positions of carbon atoms in acrystal structure of silicon carbide.

Nitrogen substitutes carbon atoms of a bilayer configuring an uppermostlayer of the silicon carbide layer 10. Excess silicon atoms or carbonatoms are emitted to the side of an insulating film, and the terminationelement is finally bonded to the silicon carbide layer 10 in atri-coordination manner. A nitrogen atom is at the position of thecarbon atom in the crystal structure of silicon carbide. A part of theoutermost silicon is absorbed into the side of the gate insulating layer28. A nitrogen atom is tri-coordinated with the silicon atoms of thesilicon carbide layer 10.

A nitrogen concentration at the peak of the nitrogen concentrationdistribution is, for example, equal to or more than 1×10²¹ cm⁻³ andequal to or less than 4×10²³ cm⁻³. In order to ensure the termination,the peak nitrogen concentration is preferably equal to or more than1×10²² cm⁻³. On the other hand, if there is excess nitrogen, this causescharge trapping, so that the peak nitrogen concentration is preferablyequal to or less than 1×10²³ cm⁻³. Typically, the peak nitrogenconcentration is about 5.0×10²² cm⁻³, that is, 5.0×10²² cm⁻³±5%. Whenthe nitrogen concentration at the peak is in the above range, goodcharacteristics in which there is no charge trapping are exhibited, forexample.

A nitrogen area density at the interface is preferably equal to or morethan 1×10¹⁴ cm⁻² and equal to or less than 2.5×10¹⁵ cm⁻². Typically, thenitrogen area density is about 1.4×10¹⁵ cm⁻², that is, 1.4×10¹⁵ cm⁻²±5%.When the nitrogen area density is in the above range, goodcharacteristics in which there is no charge trapping are exhibited, forexample.

The carbon concentration distribution decreases from the interfacetermination region 40 toward the gate insulating layer 28. The carbonconcentration at the position X is equal to or less than 1×10¹⁸ cm⁻³.The nitrogen concentration is preferably equal to or less than 1×10¹⁷cm⁻³ and more preferably equal to or less than 1×10¹⁶ cm⁻³.

A concentration at the position X of a complex including the carbonatoms bonded to the oxygen atoms and the nitrogen atoms bonded to theoxygen atoms is, for example, equal to or less than 1×10¹⁸ cm⁻³. Thenitrogen concentration is preferably equal to or less than 1×10¹⁷ cm⁻³and more preferably equal to or less than 1×10¹⁶ cm⁻³.

The gate electrode 30 is provided on the gate insulating layer 28. Thegate electrode 30 sandwiches the gate insulating layer 28 with thesilicon carbide layer 10. The gate electrode 30 sandwiches the gateinsulating layer 28 with the drift region 14. The gate electrode 30sandwiches the gate insulating layer 28 with the p-well region 16.

The gate electrode 30 is, for example, polycrystalline silicon includingn-type impurities or p-type impurities.

The interlayer insulating film 32 is formed on the gate electrode 30.The interlayer insulating film 32 is, for example, a silicon oxide film.

The source electrode 34 is electrically connected to the source region18 and the p-well contact region 20. The source electrode 34 alsofunctions as a p-well electrode for applying an electric potential tothe p-well region 16.

The source electrode 34 is formed of, for example, a stacked layer of abarrier metal layer of nickel (Ni) and a metal layer of aluminum on thebarrier metal layer. The barrier metal layer of nickel and the siliconcarbide layer may react to form nickel silicide (NiSi, Ni₂Si, or thelike). The barrier metal layer of nickel and the metal layer of aluminummay form an alloy by reaction.

The drain electrode 36 is provided on the side of the silicon carbidelayer 10 opposite to the source electrode 34, that is, the back side.The drain electrode 36 is, for example, nickel. Nickel may react withthe drain region 12 to form nickel silicide (NiSi, Ni₂Si, or the like).

In the first embodiment, the n-type impurity is, for example, nitrogenor phosphorus. Arsenic (As) or antimony (Sb) can also be applied as then-type impurity.

Further, in the first embodiment, the p-type impurity is, for example,aluminum. Boron (B), gallium (Ga), and indium (In) can also be appliedas the p-type impurity.

Next, an example of a method for manufacturing the semiconductor deviceaccording to the first embodiment will be described.

The method for manufacturing the semiconductor device according to thefirst embodiment includes forming a silicon oxide film on a face of asilicon carbide layer and performing first heat treatment in anatmosphere including carbon dioxide gas and at least one oxidizing gasselected from the group consisting of nitrogen oxide gas, oxygen gas,and water vapor.

FIG. 4 is a process flow diagram of the method for manufacturing thesemiconductor device according to the first embodiment.

As shown in FIG. 4, the method for manufacturing the semiconductordevice according to the first embodiment includes silicon carbide layerpreparation (step S100), p-type impurity ion implantation (step S101),n-type impurity ion implantation (step S102), p-type impurity ionimplantation (step S103), silicon oxide film formation (step S104),first heat treatment (step S105), gate electrode formation (step S106),interlayer insulating film formation (step S107), source electrodeformation (step S108), and drain electrode formation (step S109).

In step S100, the silicon carbide layer 10 is prepared. The siliconcarbide layer 10 includes the n⁺-type drain region 12 and the n⁻-typedrift region 14. The drift region 14 is formed on the drain region 12 byan epitaxial growth method, for example.

The drain region 12 includes nitrogen as n-type impurities. An n-typeimpurity concentration of the drain region 12 is, for example, equal toor more than 1×10¹⁸ cm⁻³ and equal to or less than 1×10²¹ cm⁻³.

The drift region 14 includes nitrogen as n-type impurities. The n-typeimpurity concentration of the drift region 14 is, for example, equal toor more than 1×10¹⁵ cm⁻³ and equal to or less than 2×10¹⁶ cm⁻³. Athickness of the drift region 14 is, for example, equal to or more than5 μm and equal to or less than 100 μm.

In step S101, first, a first mask material is formed by patterning usingphotolithography and etching. Then, by using the first mask material asan ion implantation mask, aluminum to be p-type impurities ision-implanted into the drift region 14. The p-well region 16 is formedby ion implantation.

In step S102, first, a second mask material is formed by patterningusing photolithography and etching. Then, by using the second maskmaterial as an ion implantation mask, phosphorus to be n-type impuritiesis ion-implanted into the drift region 14 to form the source region 18.

In step S103, a third mask material is formed by patterning usingphotolithography and etching. By using the third mask material as an ionimplantation mask, aluminum to be p-type impurities is ion-implantedinto the drift region 14 to form the p-well contact region 20.

In step S104, a silicon oxide film is formed on the silicon carbidelayer 10. The silicon oxide film finally becomes the gate insulatinglayer 28.

The silicon oxide film is formed by vapor phase growth. The siliconoxide film is formed by, for example, a chemical vapor deposition method(CVD method) or a physical vapor deposition method (PVD method). Thesilicon oxide film is a deposited film. The thickness of the siliconoxide film is, for example, equal to or more than 30 nm and equal to orless than 100 nm. The thickness of the silicon oxide film is, forexample, equal to or more than 40 nm and equal to or less than 50 nm.

The silicon oxide film is, for example, a silicon oxide film formed bythe CVD method using tetraethyl orthosilicate (TEOS) as source gas.

In step S105, first heat treatment is performed. The first heattreatment is performed in an atmosphere including nitrogen oxide gas(NOx) and carbon dioxide gas (CO₂). The nitrogen oxide gas is an exampleof oxidizing gas. The nitrogen oxide gas is, for example, nitricmonoxide gas (NO). Further, the nitrogen oxide gas is, for example,dinitrogen monoxide gas (N₂O).

For example, heat treatment is performed by supplying nitrogen oxide gas(NOx) and carbon dioxide gas (CO₂) to a reaction furnace containing thesilicon carbide layer 10.

A temperature of the first heat treatment is, for example, equal to ormore than 1050° C. and equal to or less than 1450° C.

A partial pressure of the carbon dioxide gas in the atmosphere of thefirst heat treatment is, for example, equal to or more than 10% andequal to or less than 50%.

A partial pressure of the nitrogen oxide gas in the atmosphere of thefirst heat treatment is, for example, equal to or more than 10% andequal to or less than 50%.

A ratio of the partial pressure of the nitrogen oxide gas to the partialpressure of the carbon dioxide gas in the atmosphere of the first heattreatment is, for example, equal to or more than 0.8 and equal to orless than 1.2.

By the first heat treatment, the interface termination region 40 isformed at the interface between the silicon carbide layer 10 and thesilicon oxide film. By the first heat treatment, a silicon oxide filmwith reduced carbon defects is formed.

The first heat treatment also functions as densification annealing ofthe silicon oxide film. By the first heat treatment, the silicon oxidefilm becomes a high-density film.

In step S106, the gate electrode 30 is formed on the gate insulatinglayer 28. The gate electrode 30 is, for example, polycrystalline siliconincluding n-type impurities or p-type impurities.

In step S107, the interlayer insulating film 32 is formed on the gateelectrode 30. The interlayer insulating film 32 is, for example, asilicon oxide film.

In step S108, the source electrode 34 is formed. The source electrode 34is formed on the source region 18 and the p-well contact region 20. Thesource electrode 34 is formed by sputtering of nickel (Ni) and aluminum(Al), for example.

In step S109, the drain electrode 36 is formed. The drain electrode 36is formed on the back side of the silicon carbide layer 10. The drainelectrode 36 is formed by sputtering of nickel, for example.

The MOSFET 100 shown in FIG. 1 is formed by the above manufacturingmethod.

Next, functions and effects of the semiconductor device and the methodfor manufacturing the semiconductor device according to the firstembodiment will be described.

When the MOSFET is formed using silicon carbide, there is a problem thatcarrier mobility decreases. One of factors causing the decrease in thecarrier mobility is considered to be an interface state between thesilicon carbide layer and the gate insulating layer. It is consideredthat the interface state is caused by the dangling bond existing on theface of the silicon carbide layer.

The MOSFET 100 according to the first embodiment includes the interfacetermination region 40 in which nitrogen is segregated between thesilicon carbide layer 10 and the gate insulating layer 28. By theinterface termination region 40, the dangling bond is reduced.Therefore, a MOSFET in which the decrease in the carrier mobility issuppressed is realized.

Further, when the MOSFET is formed using silicon carbide, there is aproblem that the carrier mobility decreases or a threshold voltagechanges. Further, there is a problem that a leakage current of the gateinsulating layer increases or reliability of the gate insulating layerdecreases. One of factors causing the above problems is considered to becarbon defects existing in the gate insulating layer.

The carbon defects are considered to be the factor causing the aboveproblems by forming trap levels in the gate insulating layer.

There are various forms in the carbon defects. The carbon defects are,for example, a double bond between carbon atoms, tri-coordination carbonin which three silicon atoms are coordinated, a structure in whichoxygen atoms are double-bonded to carbon atoms, and the like. It hasbeen clarified by the first principle calculation by the inventors thatthese carbon defects form trap levels due to Pz orbitals. These carbondefects are formed by the introduction of carbon atoms into oxygen sitesof silicon oxide.

Another form of the carbon defects is, for example, a complex includingcarbon atoms bonded to oxygen atoms and nitrogen atoms bonded to theoxygen atoms. The complex is a C—O—N bond.

FIG. 5 is a diagram illustrating the carbon defects. FIG. 5 shows acomplex including a carbon atom bonded to an oxygen atom and a nitrogenatom bonded to the oxygen atom. FIG. 5 shows a C—O—N bond. The carbonatom and the nitrogen atom of the C—O—N bond are introduced into thesilicon sites of silicon oxide.

FIG. 6 is a process flow diagram of a method for manufacturing asemiconductor device according to a comparative example. In the methodfor manufacturing the semiconductor device according to the comparativeexample, heat treatment (step S905) is performed instead of the firstheat treatment (step S105) in the method for manufacturing thesemiconductor device according to the first embodiment.

In step S905, the heat treatment is performed. The heat treatment isperformed in an atmosphere including nitrogen oxide gas (NOx). Thenitrogen oxide gas is, for example, nitric monoxide gas (NO). Further,the nitrogen oxide gas is, for example, dinitrogen monoxide gas (N₂O).

A temperature of the heat treatment is, for example, equal to or morethan 1050° C. and equal to or less than 1450° C.

The heat treatment in step S905 does not include carbon dioxide gas(CO₂) as heat treatment atmosphere gas. For example, the heat treatmentis heat treatment at 1200° C. in N₂ diluted NO 50% atmosphere gas. Sincethe gas generated by substrate oxidation (CO gas or CO₂ gas) isdischarged to an abatement system, the heat treatment does not includean effective amount (equal to or more than 10% in a partial pressure) ofcarbon dioxide as atmosphere gas during the heat treatment. If it isdesired to introduce an effective amount of carbon dioxide, it isnecessary to intentionally introduce carbon dioxide as the heattreatment atmosphere gas.

By the heat treatment in step S905, the interface termination region isformed at the interface between the silicon carbide layer and thesilicon oxide film.

FIG. 7 is a diagram showing an element concentration distribution of thesemiconductor device according to the comparative example. Thesemiconductor device according to the comparative example is a MOSFETmanufactured by the manufacturing method shown in FIG. 6.

FIG. 7 is a diagram showing element concentration distributions in thegate insulating layer, the interface termination region, and the siliconcarbide layer. FIG. 7 shows concentration distributions of nitrogen andcarbon.

The nitrogen concentration distribution has a peak in the interfacetermination region. A peak nitrogen concentration is, for example, equalto or more than 1×10²¹ cm⁻³ and less than 1×10²² cm⁻³. Nitrogen issegregated at the interface between the silicon carbide layer and thegate insulating layer.

The nitrogen concentration at a position X to be 1 nm away from the peakof the nitrogen concentration distribution to the side of the gateinsulating layer is higher than 1×10¹⁸ cm⁻³. As shown in FIG. 7,although the nitrogen concentration gradually decreases from theinterface, a shoulder structure is formed and a large amount of nitrogenis distributed in the insulating film.

The carbon concentration distribution decreases from the interfacetermination region toward the gate insulating layer. The carbonconcentration at the position X is higher than 1×10¹⁸ cm⁻³. It isconsidered that carbon is diffused into the gate insulating film,because the heat treatment in step S905 is accompanied by the substrateoxidation. As shown in FIG. 7, although the carbon concentrationgradually decreases from the interface, a shoulder structure is formedand a large amount of carbon is distributed in the insulating film.

The MOSFET according to the comparative example includes the interfacetermination region in which nitrogen is segregated. Therefore, similarlyto the MOSFET 100 according to the first embodiment, the decrease in thecarrier mobility is suppressed.

The MOSFET according to the comparative example has a higherconcentration of carbon or nitrogen in the gate insulating layer thanthat in the MOSFET 100 according to the first embodiment. Carbon in thegate insulating layer forms carbon defects. Further, nitrogen in thegate insulating layer forms, for example, a C—O—N bond.

It is considered that carbon in the gate insulating layer is derivedfrom carbon released from the silicon carbide layer, when the face ofthe silicon carbide layer is oxidized by the nitrogen oxide gas.Further, it is considered that nitrogen in the nitrogen oxide gas isbonded to carbon released from the silicon carbide layer to form a C—O—Nbond, so that nitrogen remains in the gate insulating layer.

Therefore, in the MOSFET according to the comparative example, thedecrease in the carrier mobility, the change in the threshold voltage,the increase in the leakage current of the gate insulating layer, or thedecrease in the reliability of the gate insulating layer due to thecarbon defects in the gate insulating layer causes a problem.

In the MOSFET 100 according to the first embodiment, as shown in FIG. 3,the carbon concentration at the position X in the gate insulating layer28 is equal to or less than 1×10¹⁸ cm⁻³. Further, the nitrogenconcentration at the position X is equal to or less than 1×10¹⁸ cm⁻³.

In the MOSFET 100 according to the first embodiment, as shown in FIG. 3,the concentration of carbon or nitrogen in the gate insulating layer 28is lower than that of the MOSFET according to the comparative example.Therefore, the amount of carbon defects in the gate insulating layer 28is small. The amount of C—O—N bonds to be carbon defects includingnitrogen in the gate insulating layer 28 is also small.

Therefore, in the MOSFET 100 according to the first embodiment, thedecrease in the carrier mobility, the change in the threshold voltage,the increase in the leakage current of the gate insulating layer, or thedecrease in the reliability of the gate insulating layer due to thecarbon defects in the gate insulating layer is suppressed.

From the viewpoint of reducing the amount of carbon defects in the gateinsulating layer 28, the concentration at the position X of the complexincluding the carbon atom bonded to the oxygen atom and the nitrogenatom bonded to the oxygen atom is preferably equal to or less than1×10¹⁸ cm⁻³. In other words, the concentration at the position X of theC—O—N bond is preferably equal to or less than 1×10¹⁸ cm⁻³.

The MOSFET 100 according to the first embodiment is manufactured usingthe manufacturing method according to the first embodiment.

In the manufacturing method according to the first embodiment, after thesilicon oxide film is formed in step S104, the first heat treatment isperformed in step S105. The first heat treatment is performed in anatmosphere including carbon dioxide gas (CO₂) and nitrogen oxide gas(NOx).

By the nitrogen oxide gas, the interface termination region 40 in whichthe dangling bond on the face of the silicon carbide layer 10 isterminated by nitrogen is formed. At this time, the nitrogen atoms areintroduced into the carbon sites of the crystal structure of siliconcarbide, so that the dangling bond is terminated.

By the nitrogen oxide gas, the interface termination region 40 is formedwhile the face of the silicon carbide layer 10 is oxidized.

By the presence of the carbon dioxide gas in the atmosphere of the firstheat treatment, a reaction of Formula (1) progresses on the face of thesilicon carbide layer 10. That is, the presence of the carbon dioxidegas becomes a driving force that progresses the reaction of Formula (1)to the right.

C+CO₂→2CO   (1)

By the presence of the carbon dioxide gas in the atmosphere, an effectof extracting the carbon atoms from the carbon sites of silicon carbideis produced. Therefore, the nitrogen atoms are easily introduced intothe carbon sites of silicon carbide. In other words, the interfacetermination due to the nitrogen atoms is likely to progress.

Therefore, the first heat treatment can be performed at a lowertemperature as compared with a case where the carbon dioxide gas is notpresent in the atmosphere. By lowering the temperature of the first heattreatment, oxidation of the face of the silicon carbide layer 10 due tothe nitrogen oxide gas can be suppressed. By suppressing the oxidationof the face of the silicon carbide layer 10, the nitrogen concentrationof the interface termination region 40 can be increased.

Therefore, as compared with the manufacturing method according to thecomparative example, the nitrogen concentration of the interfacetermination region 40 can be increased in the manufacturing methodaccording to the first embodiment. The nitrogen concentration at thepeak of the nitrogen concentration distribution in the interfacetermination region 40 can be increased. By increasing the nitrogenconcentration at the peak of the nitrogen concentration distribution inthe interface termination region 40, the decrease in the carriermobility is suppressed.

From the viewpoint of suppressing the decrease in the carrier mobility,the nitrogen concentration at the peak of the nitrogen concentrationdistribution in the interface termination region 40 of the nitrogenconcentration distribution is preferably equal to or more than 1×10²²cm⁻³ and more preferably equal to or more than 5×10²² cm⁻³.

Further, the reaction of Formula (1) progresses, so that carbon releasedby the oxidation of the face of the silicon carbide layer 10 becomes COand is removed into the atmosphere. Therefore, the amount of carbonremaining in the gate insulating layer 28 decreases. As a result, thecarbon defects in the gate insulating layer 28 are reduced.

Further, the amount of carbon remaining in the gate insulating layer 28decreases, so that the amount of C—O—N bonds formed in the gateinsulating layer 28 also decreases. Therefore, the nitrogenconcentration in the gate insulating layer 28 also decreases.

According to the manufacturing method according to the first embodiment,it is possible to realize the MOSFET 100 in which the amount of carbondefects in the gate insulating layer 28 is reduced.

From the viewpoint of progressing oxidation of the silicon carbide layer10 and increasing the nitrogen concentration in the interfacetermination region 40, the temperature of the first heat treatment ispreferably equal to or more than 1050° C. and more preferably equal toor more than 1100° C.

From the viewpoint of suppressing excessive oxidation of the siliconcarbide layer 10 and increasing the nitrogen concentration in theinterface termination region 40, the temperature of the first heattreatment is preferably equal to or less than 1450° C., more preferablyequal to or less than 1350° C., further preferably equal to or less than1250° C., and most preferably equal to or less than 1150° C.

From the viewpoint of effectively removing carbon released duringoxidation of the face of the silicon carbide layer 10, the partialpressure of the carbon dioxide gas in the atmosphere of the first heattreatment is preferably equal to or more than 10%, more preferably equalto or more than 20%, and further preferably equal to or more than 30%.

From the viewpoint of progressing oxidation of the silicon carbide layer10 and increasing the nitrogen concentration in the interfacetermination region 40, the partial pressure of the nitrogen oxide gas inthe atmosphere of the first heat treatment is preferably equal to ormore than 10%, more preferably equal to or more than 20%, and furtherpreferably equal to or more than 30%.

The ratio of the partial pressure of the nitrogen oxide gas to thepartial pressure of the carbon dioxide gas in the atmosphere of thefirst heat treatment is preferably equal to or more than 0.8 and equalto or less than 1.2. When carbon at the substrate interface is extractedwith carbon dioxide, the presence of nitrogen in the vicinityfacilitates the progress of nitrogen substitution of carbon. Therefore,the presence of carbon dioxide and nitrogen oxide in the same amount iseffective for improving characteristics of the interface. Further, sincecarbon in the insulating film is extracted with carbon dioxide andtreated with oxygen, the presence of carbon dioxide and oxidant in thesame amount is effective for improving characteristics of the insulatingfilm.

As described above, according to the first embodiment, the semiconductordevice and the method for manufacturing the semiconductor device thatreduce the amount of carbon defects in the insulating layer arerealized.

Second Embodiment

A method for manufacturing a semiconductor device according to a secondembodiment is different from the method for manufacturing thesemiconductor device according to the first embodiment in that a siliconoxide film is formed by thermal oxidation in an atmosphere includingoxygen gas or water vapor and carbon dioxide gas.

Hereinafter, description of contents overlapping with those of the firstembodiment will be partially omitted.

Hereinafter, a case where oxidizing gas is oxygen gas will be describedas an example.

FIG. 8 is a process flow diagram of the method for manufacturing thesemiconductor device according to the second embodiment. In the methodfor manufacturing the semiconductor device according to the secondembodiment, instead of forming the silicon oxide film by vapor phasegrowth (step S104) in the method for manufacturing the semiconductordevice according to the first embodiment, the silicon oxide film isformed by the thermal oxidation in an atmosphere including oxygen gasand carbon dioxide gas (step S204).

In step S204, the silicon oxide film is formed by the thermal oxidationin an atmosphere including oxygen gas (O₂) and carbon dioxide gas (CO₂).

For example, the thermal oxidation is performed by supplying the oxygengas (O₂) and the carbon dioxide gas (CO₂) to a reaction furnacecontaining a silicon carbide layer 10.

A temperature of the thermal oxidation is, for example, equal to or morethan 1050° C. and equal to or less than 1450° C.

When the silicon carbide layer 10 is thermally oxidized, the carbondioxide gas is present in the atmosphere, so that a reaction of Formula(1) progresses on a face of the silicon carbide layer 10. That is, thepresence of the carbon dioxide gas becomes a driving force thatprogresses the reaction of Formula (1) to the right.

C+CO₂→2CO   (1)

The reaction of Formula (1) progresses, so that carbon released by theoxidation of the face of the silicon carbide layer 10 becomes CO and isremoved into the atmosphere. Therefore, an amount of carbon remaining ina gate insulating layer 28 is smaller than that in a case where thecarbon dioxide gas is not included in the atmosphere. As a result, thecarbon defects in the gate insulating layer 28 are reduced.

Further, the amount of carbon remaining in the gate insulating layer 28decreases, so that the amount of C—O—N bonds formed in the gateinsulating layer 28 also decreases. Therefore, the nitrogenconcentration in the gate insulating layer 28 also decreases.

Even when the oxidizing gas is water vapor, the same functions andeffect as those in the oxygen gas are obtained.

As described above, according to the second embodiment, thesemiconductor device and the method for manufacturing the semiconductordevice that reduce the amount of carbon defects in the insulating layerare realized.

Third Embodiment

A method for manufacturing a semiconductor device according to a thirdembodiment is different from the method for manufacturing thesemiconductor device according to the first embodiment in that a siliconoxide film is formed by thermal oxidation in an atmosphere includingnitrogen oxide gas and carbon dioxide gas, and first heat treatment isnot included. Hereinafter, description of contents overlapping withthose of the first embodiment will be partially omitted.

FIG. 9 is a process flow diagram of the method for manufacturing thesemiconductor device according to the third embodiment. In the methodfor manufacturing the semiconductor device according to the thirdembodiment, instead of forming the silicon oxide film by vapor phasegrowth (step S104) in the method for manufacturing the semiconductordevice according to the first embodiment, the silicon oxide film isformed by the thermal oxidation in an atmosphere including nitrogenoxide gas and carbon dioxide gas (step S304). Further, the method formanufacturing the semiconductor device according to the third embodimentdoes not include first heat treatment (step S105) in the method formanufacturing the semiconductor device according to the firstembodiment.

In step S304, the silicon oxide film is formed by the thermal oxidationin an atmosphere including nitrogen oxide gas (NOx) and carbon dioxidegas (CO₂). By the thermal oxidation including the nitrogen oxide gas(NOx), an interface termination region 40 is formed at an interfacebetween a silicon carbide layer 10 and the silicon oxide film.

The nitrogen oxide gas is, for example, nitric monoxide gas (NO).Further, the nitrogen oxide gas is, for example, dinitrogen monoxide gas(N₂O).

For example, the thermal oxidation is performed by supplying thenitrogen oxide gas (NOx) and the carbon dioxide gas (CO₂) to a reactionfurnace containing the silicon carbide layer 10.

A temperature of the thermal oxidation is, for example, equal to or morethan 1050° C. and equal to or less than 1450° C.

A partial pressure of the carbon dioxide gas in the atmosphere of thethermal oxidation is, for example, equal to or more than 10% and equalto or less than 50%.

A partial pressure of the nitrogen oxide gas in the atmosphere of thethermal oxidation is, for example, equal to or more than 10% and equalto or less than 50%.

A ratio of the partial pressure of the nitrogen oxide gas to the partialpressure of the carbon dioxide gas in the atmosphere of the thermaloxidation is, for example, equal to or more than 0.8 and equal to orless than 1.2.

When the silicon carbide layer 10 is thermally oxidized, the carbondioxide gas is present in the atmosphere, so that a reaction of Formula(1) progresses on a face of the silicon carbide layer 10. That is, thepresence of the carbon dioxide gas becomes a driving force thatprogresses the reaction of Formula (1) to the right.

C+CO₂→2CO   (1)

The reaction of Formula (1) progresses, so that carbon released by theoxidation of the face of the silicon carbide layer 10 becomes CO and isremoved into the atmosphere. Therefore, an amount of carbon remaining ina gate insulating layer 28 is smaller than that in a case where thecarbon dioxide gas is not included in the atmosphere. Therefore, anamount of carbon defects in the gate insulating layer 28 is reduced.

Further, the amount of carbon remaining in the gate insulating layer 28decreases, so that the amount of C—O—N bonds formed in the gateinsulating layer 28 also decreases. Therefore, the nitrogenconcentration in the gate insulating layer 28 also decreases.

Further, according to the manufacturing method according to the thirdembodiment, it is possible to simultaneously form the silicon oxide filmand the interface termination region 40. Therefore, a MOSFET is easilymanufactured.

As described above, according to the third embodiment, the semiconductordevice and the method for manufacturing the semiconductor device thatreduce the amount of carbon defects in the insulating layer arerealized.

Fourth Embodiment

A method for manufacturing a semiconductor device according to a fourthembodiment is different from the method for manufacturing thesemiconductor device according to the first embodiment in that, beforefirst heat treatment, second heat treatment is further performed at atemperature equal to or more than 1050° C. and equal to or less than1450° C., in an atmosphere including nitrogen oxide gas, and the firstheat treatment is performed in an atmosphere including oxygen gas orwater vapor and carbon dioxide gas. Hereinafter, description of contentsoverlapping with those of the first embodiment will be partiallyomitted.

Hereinafter, a case where oxidizing gas of the first heat treatment isoxygen gas will be described as an example.

FIG. 10 is a process flow diagram of the method for manufacturing thesemiconductor device according to the fourth embodiment. In the methodfor manufacturing the semiconductor device according to the fourthembodiment, instead of the first heat treatment (step S105) performed inan atmosphere including nitrogen oxide gas and carbon dioxide gas in themethod for manufacturing the semiconductor device according to the firstembodiment, first heat treatment (step S405) is performed in theatmosphere including the oxygen gas and the carbon dioxide gas. Further,second heat treatment (step S415) is performed before the first heattreatment (step S405).

The method for manufacturing the semiconductor device according to thefourth embodiment is the same as a case of adding the first heattreatment (step S405) after heat treatment (step S905) in the method formanufacturing the semiconductor device according to the comparativeexample shown in FIG. 6.

In step S104, a silicon oxide film is formed on a silicon carbide layer.The silicon oxide film finally becomes the gate insulating layer 28. Thesilicon oxide film is formed by vapor phase growth.

In step S415, the second heat treatment is performed. The second heattreatment is performed in an atmosphere including nitrogen oxide gas(NOx). The nitrogen oxide gas is, for example, nitric monoxide gas (NO).Further, the nitrogen oxide gas is, for example, dinitrogen monoxide gas(N₂O).

A temperature of the heat treatment is, for example, equal to or morethan 1050° C. and equal to or less than 1450° C.

By the second heat treatment in step S415, an interface terminationregion 40 including nitrogen is formed at an interface between thesilicon carbide layer 10 and the silicon oxide film.

In step S405, the first heat treatment is performed. The first heattreatment is performed in an atmosphere including oxygen gas (O₂) andcarbon dioxide gas (CO₂).

A temperature of the first heat treatment is lower than a temperature ofthe second heat treatment. The temperature of the first heat treatmentis, for example, equal to or more than 750° C. and less than 1050° C.

A partial pressure of the carbon dioxide gas in the atmosphere of thefirst heat treatment is, for example, equal to or more than 10% andequal to or less than 50%.

A partial pressure of the oxygen gas in the atmosphere of the first heattreatment is, for example, equal to or more than 10% and equal to orless than 50%.

A ratio of the partial pressure of the oxygen gas to the partialpressure of the carbon dioxide gas in the atmosphere of the first heattreatment is, for example, equal to or more than 0.8 and equal to orless than 1.2.

By the first heat treatment, carbon defects in the silicon oxide filmare reduced.

Hereinafter, functions and effects of the method for manufacturing thesemiconductor device according to the fourth embodiment will bedescribed.

An element concentration distribution immediately after the interfacetermination region 40 is formed at the interface between the siliconcarbide layer 10 and the silicon oxide film by the second heat treatmentin step S415 is the same as that of the semiconductor device accordingto the comparative example shown in FIG. 7.

Immediately after the second heat treatment in step S415, aconcentration of carbon or nitrogen in the silicon oxide film is high.Carbon in the silicon oxide film forms carbon defects. Further, nitrogenin the silicon oxide film forms, for example, a C—O—N bond.

In the method for manufacturing the semiconductor device according tothe fourth embodiment, after the second heat treatment, the first heattreatment is performed in the atmosphere including the oxygen gas (O₂)and the carbon dioxide gas (CO₂). By the presence of the carbon dioxidegas in the atmosphere of the first heat treatment, a reaction of Formula(1) progresses in the silicon oxide film. That is, the presence of thecarbon dioxide gas becomes a driving force that progresses the reactionof Formula (1) to the right.

C+CO₂→2CO   (1)

By the presence of the carbon dioxide gas in the atmosphere, an effectof extracting carbon atoms introduced into oxygen sites of silicon oxideas carbon monoxide gas is produced. Therefore, the carbon defects in thesilicon oxide film are reduced. In particular, the amount of carbondefects formed by the introduction of the carbon atoms into the oxygensites of silicon oxide is reduced.

If oxidizing gas such as the oxygen gas is not included in theatmosphere of the first heat treatment, portions where the carbon atomsof the oxygen sites of silicon oxide are removed become oxygen vacancies(Oxide Vacancy: Vo). The oxygen vacancies in the silicon oxide film formtrap levels. Therefore, when an amount of oxygen vacancies (Vo)increases, this is not preferable because characteristics of a MOSFETare degraded.

In the method for manufacturing the semiconductor device according tothe fourth embodiment, the oxidizing gas such as the oxygen gas isincluded in the atmosphere of the first heat treatment. Therefore, theoxygen vacancies are filled with oxygen atoms, and the oxygen vacanciesformed by the reaction of Formula (1) disappear. Therefore, degradationof the characteristics of the MOSFET is suppressed.

Further, the carbon dioxide gas (CO₂) is present in the atmosphere ofthe first heat treatment, so that a reaction of Formula (2) issuppressed. That is, a large amount of carbon dioxide on the right sidemakes it difficult for the reaction of Formula (2) to progress to theright. Note that Vo:SiO₂ means SiO₂ having the oxygen vacancies (Vo).

SiO₂+CO→Vo:SiO₂+CO₂   (2)

That is, the carbon dioxide gas is present in the atmosphere of thefirst heat treatment to suppress the formation of the oxygen vacancies(Vo) in the oxide film due to carbon monoxide (CO) discharged by Formula(1). Therefore, degradation of the characteristics of the MOSFET due tothe formation of the oxygen vacancies (Vo) is suppressed.

From the viewpoint of reducing the amount of carbon defects in thesilicon oxide film, the temperature of the first heat treatment ispreferably equal to or more than 750° C., more preferably equal to ormore than 850° C., and further preferably equal to or more than 925° C.

Further, from the viewpoint of suppressing oxidation of the siliconcarbide layer 10, the temperature of the first heat treatment ispreferably less than 1050° C., more preferably equal to or less than1000° C., and further preferably equal to or less than 950° C.

Even when the oxidizing gas is water vapor, the same functions andeffect as those in the oxygen gas are obtained.

As described above, according to the fourth embodiment, thesemiconductor device and the method for manufacturing the semiconductordevice that reduce the amount of carbon defects in the insulating layerare realized.

Fifth Embodiment

A method for manufacturing a semiconductor device according to a fifthembodiment is different from the method for manufacturing thesemiconductor device according to the fourth embodiment in that firstheat treatment is performed in an atmosphere including nitrogen oxidegas and carbon dioxide gas. Hereinafter, description of contentsoverlapping with those of the fourth embodiment will be partiallyomitted.

FIG. 11 is a process flow diagram of the method for manufacturing thesemiconductor device according to the fifth embodiment. The method formanufacturing the semiconductor device according to the fifth embodimentincludes first heat treatment (step S505) performed in the atmosphereincluding the nitrogen oxide gas and the carbon dioxide gas, instead ofthe first heat treatment (step 5405) performed in an atmosphereincluding oxygen gas and carbon dioxide gas in the method formanufacturing the semiconductor device according to the fourthembodiment.

The method for manufacturing the semiconductor device according to thefifth embodiment is the same as a case of adding the first heattreatment (step S505) after heat treatment (step S905) in the method formanufacturing the semiconductor device according to the comparativeexample shown in FIG. 6.

In step S104, a silicon oxide film is formed on a silicon carbide layer.The silicon oxide film finally becomes the gate insulating layer 28. Thesilicon oxide film is formed by vapor phase growth.

In step S415, the second heat treatment is performed. The second heattreatment is performed in an atmosphere including nitrogen oxide gas(NOx). The nitrogen oxide gas is, for example, nitric monoxide gas (NO).Further, the nitrogen oxide gas is, for example, dinitrogen monoxide gas(N₂O).

A temperature of the heat treatment is, for example, equal to or morethan 1050° C. and equal to or less than 1450° C.

By the second heat treatment in step S415, an interface terminationregion 40 including nitrogen is formed at an interface between thesilicon carbide layer 10 and the silicon oxide film.

In step S505, the first heat treatment is performed. The first heattreatment is performed in an atmosphere including nitrogen oxide gas(NOx) and carbon dioxide gas (CO₂). The nitrogen oxide gas is an exampleof oxidizing gas. The nitrogen oxide gas is, for example, nitricmonoxide gas (NO). Further, the nitrogen oxide gas is, for example,dinitrogen monoxide gas (N₂O).

A temperature of the first heat treatment is lower than a temperature ofthe second heat treatment. The temperature of the first heat treatmentis, for example, equal to or more than 750° C. and less than 1050° C.

A partial pressure of the carbon dioxide gas in the atmosphere of thefirst heat treatment is, for example, equal to or more than 10% andequal to or less than 50%.

A partial pressure of the nitrogen oxide gas in the atmosphere of thefirst heat treatment is, for example, equal to or more than 10% andequal to or less than 50%.

A ratio of the partial pressure of the nitrogen oxide gas to the partialpressure of the carbon dioxide gas in the atmosphere of the first heattreatment is, for example, equal to or more than 0.8 and equal to orless than 1.2.

By the first heat treatment, carbon defects in the silicon oxide filmare reduced.

Hereinafter, functions and effects of the method for manufacturing thesemiconductor device according to the fifth embodiment will bedescribed.

An element concentration distribution immediately after the interfacetermination region 40 is formed at the interface between the siliconcarbide layer 10 and the silicon oxide film by the second heat treatmentin step S415 is the same as that of the semiconductor device accordingto the comparative example shown in FIG. 7.

Immediately after the second heat treatment in step S415, aconcentration of carbon or nitrogen in the silicon oxide film is high.Carbon in the silicon oxide film forms carbon defects. Further, nitrogenin the silicon oxide film forms, for example, a C—O—N bond.

In the method for manufacturing the semiconductor device according tothe fifth embodiment, after the second heat treatment, the first heattreatment is performed in the atmosphere including the nitrogen oxidegas (NOx) and the carbon dioxide gas (CO₂). By the presence of thecarbon dioxide gas in the atmosphere of the first heat treatment, areaction of Formula (1) progresses in the silicon oxide film. That is,the presence of the carbon dioxide gas becomes a driving force thatprogresses the reaction of Formula (1) to the right.

C+CO₂→2CO   (1)

By the presence of the carbon dioxide gas in the atmosphere, an effectof extracting carbon atoms introduced into oxygen sites of silicon oxideas carbon monoxide gas is produced. Therefore, the carbon defects in thesilicon oxide film are reduced. In particular, the amount of carbondefects formed by the introduction of the carbon atoms into the oxygensites of silicon oxide is reduced.

If oxidizing gas such as the nitrogen oxide gas (NOx) is not included inthe atmosphere of the first heat treatment, portions where the carbonatoms of the oxygen sites of silicon oxide are removed become oxygenvacancies (Oxide Vacancy: Vo). The oxygen vacancies in the silicon oxidefilm form trap levels. Therefore, when an amount of oxygen vacancies(Vo) increases, this is not preferable because characteristics of aMOSFET are degraded.

In the method for manufacturing the semiconductor device according tothe fifth embodiment, oxidizing gas such as the nitrogen oxide gas (NOx)is included in the atmosphere of the first heat treatment. Therefore,the oxygen vacancies are filled with oxygen atoms, and the oxygenvacancies formed by the reaction of Formula (1) disappear. Therefore,degradation of the characteristics of the MOSFET is suppressed.

Further, the carbon dioxide gas (CO₂) is present in the atmosphere ofthe first heat treatment, so that a reaction of Formula (2) issuppressed. That is, a large amount of carbon dioxide on the right sidemakes it difficult for the reaction of Formula (2) to progress to theright. Note that Vo:SiO₂ means SiO₂ having the oxygen vacancies (Vo).

SiO₂+CO→Vo:SiO₂+CO₂   (2)

That is, the carbon dioxide gas is present in the atmosphere of thefirst heat treatment to suppress the formation of the oxygen vacancies(Vo) in the oxide film due to carbon monoxide (CO) discharged by Formula(1). Therefore, degradation of the characteristics of the MOSFET due tothe formation of the oxygen vacancies (Vo) is suppressed.

Further, the nitrogen oxide gas (NOx) is present in the atmosphere ofthe first heat treatment, so that an amount of C—O—N bonds in thesilicon oxide film is also reduced. It is considered that this isbecause nitrogen included in the nitrogen oxide gas reacts with nitrogenhaving a C—O—N bond, diffuses as nitrogen gas, and disappears from thesilicon oxide film.

According to the method for manufacturing the semiconductor deviceaccording to the fifth embodiment, in addition to the carbon defectsformed by the introduction of the carbon atoms into the oxygen sites ofsilicon oxide, the amount of C—O—N bonds formed by the introduction ofthe carbon atoms and the nitrogen atoms into the silicon sites ofsilicon oxide can be reduced.

From the viewpoint of reducing the amount of carbon defects in thesilicon oxide film, the temperature of the first heat treatment ispreferably equal to or more than 750° C., more preferably equal to ormore than 850° C., and further preferably equal to or more than 925° C.

Further, from the viewpoint of suppressing oxidation of the siliconcarbide layer 10, the temperature of the first heat treatment ispreferably less than 1050° C., more preferably equal to or less than1000° C., and further preferably equal to or less than 950° C.

As described above, according to the fifth embodiment, the semiconductordevice and the method for manufacturing the semiconductor device thatreduce the amount of carbon defects in the insulating layer arerealized.

Sixth Embodiment

A method for manufacturing a semiconductor device according to a sixthembodiment is different from the method for manufacturing thesemiconductor device according to the first embodiment in that firstheat treatment is performed in an atmosphere including oxygen gas orwater vapor and carbon dioxide gas. Hereinafter, description of contentsoverlapping with those of the first embodiment will be partiallyomitted.

Hereinafter, a case where oxidizing gas is oxygen gas will be describedas an example.

FIG. 12 is a process flow diagram of the method for manufacturing thesemiconductor device according to the sixth embodiment. The method formanufacturing the semiconductor device according to the sixth embodimentperforms first heat treatment (step S605) in an atmosphere includingoxygen gas (O₂) and carbon dioxide gas (CO₂), instead of performingfirst heat treatment (step S105) in an atmosphere including nitrogenoxide gas (NOx) and carbon dioxide gas (CO₂) in the method formanufacturing the semiconductor device according to the firstembodiment.

In step S605, the first heat treatment is performed. The first heattreatment is performed in an atmosphere including oxygen gas (O₂) andcarbon dioxide gas (CO₂).

A temperature of the first heat treatment is, for example, equal to ormore than 1050° C. and equal to or less than 1450° C.

A partial pressure of the carbon dioxide gas in the atmosphere of thefirst heat treatment is, for example, equal to or more than 10% andequal to or less than 50%.

A partial pressure of the oxygen gas in the atmosphere of the first heattreatment is, for example, equal to or more than 10% and equal to orless than 50%.

A ratio of the partial pressure of the oxygen gas to the partialpressure of the carbon dioxide gas in the atmosphere of the first heattreatment is, for example, equal to or more than 0.8 and equal to orless than 1.2.

By the first heat treatment, a silicon oxide film with reduced carbondefects is formed.

The first heat treatment also functions as densification annealing ofthe silicon oxide film. By the first heat treatment, the silicon oxidefilm becomes a high-density film.

In the manufacturing method according to the sixth embodiment, after thesilicon oxide film is formed in step S104, the first heat treatment isperformed in step S605. The first heat treatment is performed in anatmosphere including oxygen gas (O₂) and carbon dioxide gas (CO₂).

By the oxygen gas, oxidation of a face of the silicon carbide layer 10is progressed.

By the presence of the carbon dioxide gas in the atmosphere of the firstheat treatment, a reaction of Formula (1) progresses on the face of thesilicon carbide layer 10. That is, the presence of the carbon dioxidegas becomes a driving force that progresses the reaction of Formula (1)to the right.

C+CO₂→2CO   (1)

The reaction of Formula (1) progresses, so that carbon released by theoxidation of the face of the silicon carbide layer 10 becomes CO and isremoved into the atmosphere. Therefore, the amount of carbon remainingin the gate insulating layer 28 decreases. As a result, the carbondefects in the gate insulating layer 28 are reduced. As compared with aconventional thermal oxide film, the interface is formed by a thermaloxide film having better characteristics.

By reducing the amount of carbon remaining in the gate insulating layer28, when interface nitriding treatment such as the heat treatment (NOx)of FIG. 6 or the first heat treatment (NOx+CO₂) of FIG. 8 is added, anamount of C—O—N bonds formed in the gate insulating layer 28 is alsoreduced. Therefore, the nitrogen concentration in the gate insulatinglayer 28 also decreases.

Even when the oxidizing gas is water vapor, the same functions andeffect as those in the oxygen gas are obtained.

As described above, according to the manufacturing method according tothe sixth embodiment, it is possible to realize the MOSFET in which theamount of carbon defects in the gate insulating layer 28 is reduced.

Seventh Embodiment

A semiconductor device according to a seventh embodiment is differentfrom the semiconductor device according to the first embodiment in thatthe semiconductor device is a trench gate type MOSFET including a gateelectrode in a trench. Hereinafter, description of contents overlappingwith those of the first embodiment will be partially omitted.

FIG. 13 is a schematic cross-sectional view of the semiconductor deviceaccording to the seventh embodiment. The semiconductor device accordingto the seventh embodiment is a MOSFET 200. The MOSFET 200 is a trenchgate type MOSFET including a gate electrode in a trench. Further, theMOSFET 200 is an n-channel MOSFET using electrons as carriers.

The MOSFET 200 includes a silicon carbide layer 10, a gate insulatinglayer 28 (silicon oxide layer), a gate electrode 30, an interlayerinsulating film 32, a source electrode 34, a drain electrode 36, aninterface termination region 40 (region), and a trench 50.

The silicon carbide layer 10 includes a drain region 12, a drift region14, a p-well region 16, a source region 18, and a p-well contact region20.

The trench 50 penetrates the source region 18 and the p-well region 16and reaches the drift region 14. A bottom face of the trench 50 isdisposed in the drift region 14.

In the trench 50, the gate insulating layer 28 and the gate electrode 30are provided. Side faces of the trench 50 are, for example, faces havingoff angles equal to or more than 0 degrees and equal to or less than 8degrees with respect to an m face.

As described above, according to the seventh embodiment, a semiconductordevice in which an amount of carbon defects in the gate insulating layer28 is reduced can be realized. Further, since the trench gate type isused, a channel density per unit area of a chip is increased, andon-resistance of the MOSFET is reduced.

Eighth Embodiment

A semiconductor device according to an eighth embodiment is differentfrom the semiconductor device according to the first embodiment in thata gate insulating layer is present in a termination region of a MOSFET.Hereinafter, description of contents overlapping with those of the firstembodiment will be partially omitted.

FIG. 14 is a schematic cross-sectional view of the semiconductor deviceaccording to the eighth embodiment. The semiconductor device accordingto the eighth embodiment is a MOSFET 300. The MOSFET 300 includes anelement region and a termination region provided around the elementregion. The termination region has a function of improving a breakdownvoltage of the MOSFET 300.

In the element region, for example, the MOSFET 100 according to thefirst embodiment is disposed as a unit cell.

The termination region includes a p-type RESURF region 60, a p⁺-typecontact region 62, p-type guard ring regions 64, a gate insulating layer28 (silicon oxide layer), and a field oxide film 33.

A configuration of the gate insulating layer 28 is the same as that ofthe MOSFET 100 according to the first embodiment.

The field oxide film 33 is, for example, a silicon oxide film.

An interface termination region including nitrogen (not shown) isprovided between a silicon carbide layer 10 and the gate insulatinglayer 28.

When the MOSFET 300 is turned off, a depletion layer is formed in theRESURF region 60, the guard ring regions 64, and a drift region 14between the guard ring regions 64, so that a breakdown voltage of theMOSFET 300 is improved.

However, when there is a trap level due to carbon defects in the gateinsulating layer 28, the charges are trapped in an energy level. By anelectric field of the trapped charges, a desired depletion layer may notbe formed. In this case, the breakdown voltage of the MOSFET 300 isdegraded.

According to the eighth embodiment, the amount of carbon defects in thegate insulating layer 28 is reduced.

Therefore, the trap level in the gate insulating layer 28 is reduced. Asa result, a desired depletion layer is formed and a MOSFET having astable breakdown voltage is realized.

Ninth Embodiment

An inverter circuit and a drive device according to a ninth embodimentare an inverter circuit and a drive device including the semiconductordevice according to the first embodiment.

FIG. 15 is a schematic diagram of the drive device according to theninth embodiment. A drive device 700 includes a motor 140 and aninverter circuit 150.

The inverter circuit 150 includes three semiconductor modules 150a,150b, and 150c using the MOSFET 100 according to the first embodiment asa switching element. By connecting the three semiconductor modules 150a, 150 b, and 150 c in parallel, the three-phase inverter circuit 150including three AC voltage output terminals U, V, and W is realized. Themotor 140 is driven by an AC voltage output from the inverter circuit150.

According to the ninth embodiment, characteristics of the invertercircuit 150 and the drive device 700 are improved by including theMOSFET 100 having improved characteristics.

Tenth Embodiment

A vehicle according to a tenth embodiment is a vehicle including thesemiconductor device according to the first embodiment.

FIG. 16 is a schematic diagram of the vehicle according to the tenthembodiment. A vehicle 800 according to the tenth embodiment is arailroad vehicle. The vehicle 800 includes motors 140 and an invertercircuit 150.

The inverter circuit 150 includes three semiconductor modules using theMOSFET 100 according to the first embodiment as a switching element. Byconnecting the three semiconductor modules in parallel, the three-phaseinverter circuit 150 including three AC voltage output terminals U, V,and W is realized. The motor 140 is driven by an AC voltage output fromthe inverter circuit 150. Wheels 90 of the vehicle 800 are rotated bythe motor 140.

According to the tenth embodiment, characteristics of the vehicle 800are improved by including the MOSFET 100 having improvedcharacteristics.

Eleventh Embodiment

A vehicle according to an eleventh embodiment is a vehicle including thesemiconductor device according to the first embodiment.

FIG. 17 is a schematic diagram of the vehicle according to the eleventhembodiment. A vehicle 900 according to the eleventh embodiment is anautomobile. The vehicle 900 includes a motor 140 and an inverter circuit150.

The inverter circuit 150 includes three semiconductor modules using theMOSFET 100 according to the first embodiment as a switching element. Byconnecting the three semiconductor modules in parallel, the three-phaseinverter circuit 150 including three AC voltage output terminals U, V,and W is realized.

The motor 140 is driven by an AC voltage output from the invertercircuit 150. Wheels 90 of the vehicle 900 are rotated by the motor 140.

According to the eleventh embodiment, characteristics of the vehicle 900are improved by including the MOSFET 100 having improvedcharacteristics.

Twelfth Embodiment

An elevator according to a twelfth embodiment is an elevator includingthe semiconductor device according to the first embodiment.

FIG. 18 is a schematic diagram of the elevator according to the twelfthembodiment. An elevator 1000 according to the twelfth embodimentincludes a car 610, a counter weight 612, a wire rope 614, a windingmachine 616, a motor 140, and an inverter circuit 150.

The inverter circuit 150 includes three semiconductor modules using theMOSFET 100 according to the first embodiment as a switching element. Byconnecting the three semiconductor modules in parallel, the three-phaseinverter circuit 150 including three AC voltage output terminals U, V,and W is realized.

The motor 140 is driven by an AC voltage output from the invertercircuit 150. The winding machine 616 is rotated by the motor 140 and thecar 610 is elevated.

According to the twelfth embodiment, characteristics of the elevator1000 are improved by including the MOSFET 100 having improvedcharacteristics.

As described above, in the first to eighth embodiments, the case where4H—SiC is used as the crystal structure of silicon carbide has beendescribed as an example. However, the present disclosure can be appliedto silicon carbide of other crystal structure such as 6H—SiC and 3C—SiC.

Further, in the first to eighth embodiments, the case where the gateinsulating layer 28 is provided on the silicon face or the m face of thesilicon carbide layer has been described as an example. However, thepresent disclosure can be applied to a case where the gate insulatinglayer 28 is provided on other face of the silicon carbide layer, forexample, a carbon face, an a face, a (0-33-8) face, or the like.

An oxidation rate of the silicon carbide layer depends on the planeorientation. In the first to eighth embodiments, it is preferable tooptimize the temperature of the first heat treatment or the second heattreatment according to the plane orientation.

Further, the present disclosure can be applied to an n-channel insulatedgate bipolar transistor (IGBT).

Further, the present disclosure can be applied to a p-channel MOSFET orIGBT, in addition to the n-channel MOSFET or IGBT.

Further, in the ninth to twelfth embodiments, the case where thesemiconductor device according to the present disclosure is applied tothe vehicle or the elevator has been described as an example. However,the semiconductor device according to the present disclosure can beapplied to a power conditioner of a photovoltaic power generation systemand the like, for example.

Further, in the ninth to twelfth embodiments, the case where thesemiconductor device according to the first embodiment is applied hasbeen described as an example. However, the semiconductor deviceaccording to any one of the second to eighth embodiments can be applied.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the semiconductor device, the methodfor manufacturing the semiconductor device, the inverter circuit, thedrive device, the vehicle, and the elevator described herein may beembodied in a variety of other forms; furthermore, various omissions,substitutions and changes in the form of the devices and methodsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. A semiconductor device comprising: a siliconcarbide layer; a silicon oxide layer; and a region disposed between thesilicon carbide layer and the silicon oxide layer and having a nitrogenconcentration equal to or more than 1×10²¹ cm⁻³, wherein nitrogenconcentration distribution in the silicon carbide layer, the siliconoxide layer, and the region have a peak in the region, a nitrogenconcentration at a position 1 nm away from the peak to the side of thesilicon oxide layer is equal to or less than 1×10¹⁸ cm⁻³, and a carbonconcentration at the position is equal to or less than 1×10¹⁸ cm⁻³. 2.The semiconductor device according to claim 1, wherein a nitrogenconcentration at the peak is equal to or more than 1×10²² cm⁻³.
 3. Thesemiconductor device according to claim 1, wherein a concentration atthe position of a complex including a carbon atom bonded to an oxygenatom and a nitrogen atom bonded to the oxygen atom is equal to or lessthan 1×10¹⁸ cm⁻³.
 4. The semiconductor device according to claim 1,further comprising a gate electrode, the silicon oxide layer beinginterposed between the gate electrode and the silicon carbide layer. 5.An inverter circuit comprising the semiconductor device according toclaim
 1. 6. A drive device comprising the semiconductor device accordingto claim
 1. 7. A vehicle comprising the semiconductor device accordingto claim
 1. 8. An elevator comprising the semiconductor device accordingto claim
 1. 9. A method for manufacturing a semiconductor device,comprising: forming a silicon oxide film on a silicon carbide layer; andperforming first heat treatment in an atmosphere including carbondioxide gas and at least one oxidizing gas selected from the groupconsisting of nitrogen oxide gas, oxygen gas, and water vapor.
 10. Themethod according to claim 9, wherein the silicon oxide film is formed byvapor phase growth.
 11. The method according to claim 10, wherein thefirst heat treatment is performed at a temperature equal to or more than1050° C. and equal to or less than 1450° C., in an atmosphere includingnitrogen oxide gas and carbon dioxide gas.
 12. The method according toclaim 10, wherein the first heat treatment is performed at a temperatureequal to or more than 1050° C. and equal to or less than 1450° C., in anatmosphere including oxygen gas or water vapor and carbon dioxide gas.13. The method according to claim 10, further comprising: performingsecond heat treatment before the performing the first heat treatment ata temperature equal to or more than 1050° C. and equal to or less than1450° C., in an atmosphere including nitrogen oxide gas; wherein thefirst heat treatment is performed at a temperature equal to or more than750° C. and less than 1050° C.
 14. The method according to claim 13,wherein the first heat treatment is performed in an atmosphere includingnitrogen oxide gas and carbon dioxide gas.
 15. The method according toclaim 9, wherein the silicon oxide film is formed by thermal oxidationin an atmosphere including oxygen gas and carbon dioxide gas, and thefirst heat treatment is performed at a temperature equal to or more than1050° C. and equal to or less than 1450° C., in an atmosphere includingnitrogen oxide gas and carbon dioxide gas.
 16. The method according toclaim 9, wherein a partial pressure of the carbon dioxide gas in theatmosphere of the first heat treatment is equal to or more than 10%. 17.The method according to claim 9, wherein a partial pressure of theoxidizing gas in the atmosphere of the first heat treatment is equal toor more than 10%.
 18. The method according to claim 9, wherein athickness of the silicon oxide film is equal to or more than 30 nm andequal to or less than 100 nm.
 19. The method according to claim 9,further comprising: forming a gate electrode on the silicon oxide film.20. A method for manufacturing a semiconductor device, comprising:forming a silicon oxide film on a silicon carbide layer by thermaloxidation, in an atmosphere including carbon dioxide gas and at leastone oxidizing gas selected from the group consisting of nitrogen oxidegas, oxygen gas, and water vapor.
 21. The method according to claim 20,wherein the thermal oxidation is performed at a temperature equal to ormore than 1050° C. and equal to or less than 1450° C., in an atmosphereincluding nitrogen oxide gas and carbon dioxide gas.
 22. The methodaccording to claim 20, wherein a partial pressure of the carbon dioxidegas in the atmosphere is equal to or more than 10%.
 23. The methodaccording to claim 20, wherein a partial pressure of the oxidizing gasin the atmosphere is equal to or more than 10%.
 24. The method accordingto claim 20, wherein a thickness of the silicon oxide film is equal toor more than 30 nm and equal to or less than 100 nm.
 25. The methodaccording to claim 20, further comprising: forming a gate electrode onthe silicon oxide film.